Antibodies reactive with the mouse mammary tumor virus in sera of breast cancer patients

Mouse Mammary Tumour Virus (MMTV) in Human Breast Cancer—The Value of Bradford Hill Criteria

For many decades, the betaretrovirus, mouse mammary tumour virus (MMTV), has been a causal suspect for human breast cancer. In recent years, substantial new evidence has been developed. Based on this evidence, we hypothesise that MMTV has a causal role. We have used an extended version of the classic A. Bradford Hill causal criteria to assess the evidence. 1. Identification of MMTV in human breast cancers: The MMTV 9.9 kb genome in breast cancer cells has been identified. The MMTV genome in human breast cancer is up to 98% identical to MMTV in mice. 2. Epidemiology: The prevalence of MMTV positive human breast cancer is about 35 to 40% of breast cancers in Western countries and 15 to 20% in China and Japan. 3. Strength of the association between MMTV and human breast cancer: Consistency—MMTV env gene sequences are consistently five-fold higher in human breast cancer as compared to benign and normal breast controls. 4. Temporality (timing) of the association: MMTV has been identified in benign and normal breast tissues up to 10 years before the development of MMTV positive breast cancer in the same patient. 5. Exposure: Exposure of humans to MMTV leads to development of MMTV positive human breast cancer. 6. Experimental evidence: MMTVs can infect human breast cells in culture; MMTV proteins are capable of malignantly transforming normal human breast epithelial cells; MMTV is a likely cause of biliary cirrhosis, which suggests a link between MMTV and the disease in humans. 7. Coherence—analogy: The life cycle and biology of MMTV in humans is almost the same as in experimental and feral mice. 8. MMTV Transmission: MMTV has been identified in human sputum and human milk. Cereals contaminated with mouse fecal material may transmit MMTV. These are potential means of transmission. 9. Biological plausibility: Retroviruses are the established cause of human cancers. Human T cell leukaemia virus type I (HTLV-1) causes adult T cell leukaemia, and human immunodeficiency virus infection (HIV) is associated with lymphoma and Kaposi sarcoma. 10. Oncogenic mechanisms: MMTV oncogenesis in humans probably differs from mice and may involve the enzyme APOBEC3B. Conclusion: In our view, the evidence is compelling that MMTV has a probable causal role in a subset of approximately 40% of human breast cancers.

Prevalence and characteristics of mouse mammary tumor virus-like virus associated breast cancer in China

BACKGROUND

Despite extensive molecular epidemiological studies, the prevalence and characteristics of Mouse Mammary Tumor Virus-Like Virus (MMTV-LV) in Chinese women breast cancer are still unclear. Besides, the prevalence of MMTV-LV in women breast cancer tissue varies in different countries and its dependent factors remain inconclusive.

METHODS

In the first part of the study, a case-control study was performed. 119 breast cancer samples (84 from Northern China and 35 from Southern China) and 50 breast fibroadenoma specimens were collected from Chinese women patients. MMTV-like env sequence and the homology to MMTV env gene were analysed by semi-nested polymerase chain reaction (PCR). We also explored the association of MMTV-LV prevalence with sample sources (Southern and Northern China) and patients' clinicopathological characteristics. To investigate the dependent factors of the prevalence of MMTV-LV in breast cancer worldwide, a meta-analysis was conducted in the second part of the study.

RESULTS

We found that the prevalence of MMTV-LV was much higher in breast cancer tissues (17.65%) than that in breast fibroadenoma specimens (4.00%) (P < 0.05). MMTV-LV prevalence in Chinese women breast cancer tissues was significantly different between Southern China (5.71%) and Northern China (22.62%) (P < 0.05). The prevalence of MMTV-LV also associates significantly with expression of HER2, but shows no significant correlation with other parameters. In the meta-analysis, we found that MMTV-LV prevalence in breast cancer tissue was dependent on the distribution of M. domesticus mouse (M. d), M. musculus mouse (M.m) and M.castaneus mouse (M.c) worldwide (P < 0.05).

CONCLUSION

The distribution of house mice may be a crucial environmental factor that explains the geographic differences in human breast cancer incidence. Our findings may provide a potential avenue of prevention, diagnosis and treatment for breast cancer.

Evidence for a causal role by mouse mammary tumour-like virus in human breast cancer

We have reviewed the evidence relevant to mouse mammary tumour viruses (MMTV) and human breast cancer. The prevalence of MMTV- like gene sequences is 15-fold higher in human breast cancer than in normal human breast tissue controls and is present in up to 40% of human breast cancers. MMTV-like gene sequences can be identified in benign breast tissues 1–11 years before the development of positive MMTV-like breast cancer in the same women. The prevalence of MMTV antibodies in sera from women with breast cancer is 5-fold higher than in normal women. MMTV can infect human breast epithelial cells and integrate at random into the human genome located in those cells. MMTV-like gene sequences are present in human milk from normal lactating women and with increased prevalence in milk from women at risk of breast cancer. MMTV-like virus associated human breast cancer has strikingly similar features to MMTV-associated mouse mammary tumours. These features include almost identical nucleotide sequences and structure of the MMTV genome, histology, superantigen expression, MMTV infection of B and T lymphocytes and hormone dependence. MMTV-like gene sequences have also been identified in dogs, cats, monkeys, mice and rats. Saliva has been identified as the most plausible means of transmission from human to human and possibly from dogs to humans. The evidence meets the classic causal criteria. A causal role for MMTV-like viruses in human breast cancer is highly likely.

MMTV mouse models and the diagnostic values of MMTV-like sequences in human breast cancer

Mouse mammary tumor virus (MMTV) long terminal repeat (LTR)-driven transgenic mice are excellent models for breast cancer as they allow for the targeted expression of various oncogenes and growth factors in neoplastic transformation of mammary glands. Numerous MMTV-LTR-driven transgenic mouse models of breast cancer have been created in the past three decades, including MMTV-neu/ErbB2, cyclin D1, cyclin E, Ras, Myc, int-1 and c-rel. These transgenic mice develop mammary tumors with different latency, histology and invasiveness, reflecting the oncogenic pathways activated by the transgene. Recently, homologous sequences of the env gene of MMTV have been identified in approximately 40% of human breast cancers, but not in normal breast or other types of cancers, suggesting possible involvement of mammary tumor virus in human breast carcinogenesis. Accumulating evidence demonstrates the association of MMTV provirus with progesterone receptor, p53 mutations and advanced-stage breast cancer. Thus, the detection of MMTV-like sequences may have diagnostic value to predict the clinical outcome of breast cancer patients.

Human RNA "rumor" viruses: the search for novel human retroviruses in chronic disease

Retroviruses are an important group of pathogens that cause a variety of diseases in humans and animals. Four human retroviruses are currently known, including human immunodeficiency virus type 1, which causes AIDS, and human T-lymphotropic virus type 1, which causes cancer and inflammatory disease. For many years, there have been sporadic reports of additional human retroviral infections, particularly in cancer and other chronic diseases. Unfortunately, many of these putative viruses remain unproven and controversial, and some retrovirologists have dismissed them as merely "human rumor viruses." Work in this field was last reviewed in depth in 1984, and since then, the molecular techniques available for identifying and characterizing retroviruses have improved enormously in sensitivity. The advent of PCR in particular has dramatically enhanced our ability to detect novel viral sequences in human tissues. However, DNA amplification techniques have also increased the potential for false-positive detection due to contamination. In addition, the presence of many families of human endogenous retroviruses (HERVs) within our DNA can obstruct attempts to identify and validate novel human retroviruses. Here, we aim to bring together the data on "novel" retroviral infections in humans by critically examining the evidence for those putative viruses that have been linked with disease and the likelihood that they represent genuine human infections. We provide a background to the field and a discussion of potential confounding factors along with some technical guidelines. In addition, some of the difficulties associated with obtaining formal proof of causation for common or ubiquitous agents such as HERVs are discussed.

Viruses and human breast cancer

There are well-established risk factors for breast cancer, most of which relate to estrogens and growth hormones in females. These include early-age menarche, late-age menopause, postmenopausal obesity and use of hormone therapy. However, these factors do not account for the sixfold difference in breast cancer incidence and mortality between countries and the fact that these differences dramatically lessen after migration; nor do they account for male breast cancer. Accordingly, hormone-responsive viruses have become major suspects as etiological agents for human breast cancer. Human papillomaviruses, mouse mammary tumor virus and Epstein-Barr virus are the prime candidate viruses as causes of human breast cancer. Human papillomaviruses and the mouse mammary tumor virus have hormone responsive elements that appear to be associated with enhanced replication of these viruses in the presence of corticosteroid and other hormones. This biological phenomenon is particularly relevant because of the hormone dependence of breast cancer. Viral genetic material for each of these candidate viruses has been identified by polymerase chain reaction in breast tumors but rarely in normal breast tissue controls. Pooled data from controlled studies show substantial odds ratios for the presence of viral genetic material in breast tumors compared with normal controls. These and additional data provide substantial, but not conclusive, evidence that human papillomavirus, the mouse mammary tumor virus and Epstein-Barr virus may have a role in the etiology of human breast cancer. If conclusive evidence for a role of these viruses in breast carcinogenesis can be developed, there is a practical possibility of primary prevention.

Mouse Mammary Tumor Virus Env–Derived Peptide Associates with Nucleolar Targets in Lymphoma, Mammary Carcinoma, and Human Breast Cancer

We have previously shown that the leader peptide (p14) of the Env-precursor of mouse mammary tumor virus is translocated into the nucleoli of murine T cell lymphomas that harbor this virus. Using a polyclonal antibody against recombinant p14, we show here that p14 is also localized to the nucleoli of murine mammary carcinomas and some human breast cancer samples. Affinity purification studies define a number of proteins, mostly nucleolar, that bind p14. Taken together, these findings point towards a more general involvement of p14 in lymphomagenesis and mammary carcinogenesis.

Breast cancer hypothesis: a single cause for the majority of cases

STUDY OBJECTIVE

The main cause of breast cancer remains unknown. Numerous causal factors or predisposing conditions have been proposed, but account for only a small percentage of the total disease. The current search for multiple causes is unavailing. This report explores whether any single aetiological agent may be responsible for the majority of cases, and attempts to define its properties.

METHODS

Examination of all relevant epidemiological and biological evidence.

MAIN RESULTS

Genetic inheritance is not the main cause of breast cancer because most cases are sporadic, there is a low prevalence of family history, and genetically similar women have differing rates after migration. Environmental exposure, such as pollution by industrialisation, is not a major cause, as deduced from a spectrum of epidemiological data. The possibility of infection as cause is not persuasive as there is no direct biological evidence and no epidemiological support. Oestrogen status is closely related to breast cancer risk, but there are numerous inconsistencies and paradoxes. It is suggested that oestrogens are not the proximate agent but are promoters acting in concert with the causal agent. Dietary factors, and especially fat, are associated with the aetiology of breast cancer as shown by intervention and ecological correlation studies, but the evidence from case-control and cohort studies is inconsistent and contradictory.

CONCLUSIONS

The hypothesis that best fits the epidemiological data is that dietary fat is not itself the causal agent, but produces depletion of an essential factor that is normally protective against the development of breast cancer. Many of the observed inconsistencies in the epidemiology are explainable if deficiency of this agent is permissive for breast cancer to develop. Some properties of the putative agent are outlined, and research investigations proposed.

No significant correlation between specific antibodies to mouse mammary tumour virus and human cancer

To study the possible involvement of mouse mammary tumour virus (MMTV) related agent in human cancer we analysed 300 samples of human sera for the presence of antibodies to MMTV structural proteins. All sera were tested by immunoblotting to achieve high specificity. Out of 300 sera, 22 reacted with transframe protein p30, 16 with the ribonucleoprotein p14, six with the envelope glycoprotein gp52 and three with the major core protein p27. Reactivities to p30 and p14 were observed in sera from cancer patients and healthy controls; reactivities to p27 and gp52 predominated in sera of cancer patients. Sera frequently reacted with a 42 kDa protein which is a cellular contaminant of the virus.

Natural killer cell activity in women at "high risk" for breast cancer, with and without benign breast syndrome

This study is an analysis of natural killer cell (NK) function in 155 women repeatedly tested over a 5-year period while attending breast screening clinics because of one or more of the following risk factors: family history-breast cancer in a close female relative (relative risk = 1.2-9); personal history-early menarche, non-parity, late menopause, etc. (relative risk = 1.3-3); clinical benign breast syndrome-localized and diffuse (relative risk = 2-4). Contrary to expectations, the high-risk group as a whole had significantly higher than normal relative NK function vs K562 (1.21 +/- 0.06 vs 1.00 +/- 0.06) (p less than 0.02). Division into subgroups showed that the NK activities in patients with positive family histories, personal histories, or both, were exactly the same as normal values and that the increased NK function in the high-risk group as a whole was due to those donors with benign breast syndrome (BBS). This group was also subdivided and the results were compared with the high-risk patients with no BBS. The NK activity of the group having diffuse BBS (1.67 +/- 0.05, n = 32) was significantly higher than that of the "No BBS" group (1.07 +/- 0.07, n = 102) (p less than 0.025). A paired "t'-test performed on data from 7 patients who had no BBS and diffuse BBS at different times showed a significant difference of p less than 0.001, suggesting that the elevated NK activity is a reaction to the hormonal factors which cause this condition.

Murine mammary tumor virus related antigen in human male mammary carcinoma

An antigen immunologically related to mouse mammary tumor virus (MuMTV) and the major envelope glycoprotein, gp52 of MuMTV, was identified in tissue sections of human male and female mammary carcinomas using the peroxidase-antiperoxidase technique. The specificity of the reaction was established by absorption studies. Positive reactions with the gp52 antiserum were seen in mouse and human mammary carcinomas, but not in normal mammary tissues, mammary tissues with benign diseases and in other primary malignant neoplasms. Almost all (32/36, 89%) male mammary carcinomas were positive for the gp52 related antigen. A lesser proportion of tumors among female patients (14/50, 28%) were positive. The gp52 positive tumors were significantly larger than the gp52 negative tumors in female patients (P less than 0.05). Gp52 positive tumors were also larger than gp52 negative tumors in male patients, but the difference was not statistically significant. Gp52 reactivity was also detected in metastatic mammary carcinoma in axillary lymph nodes of male and female patients. The presence of gp52 related antigen was not apparently related to tumor grade or lymphocytic infiltrate in the primary tumor. The data do not permit a firm conclusion regarding nodal status in men; no correlation of gp52 activity and nodal status in women was evident. These results indicate that mammary carcinomas in men as well as in women have an antigen related immunologically to MuMTV gp52. Other than tumor size, the antigen seems to be unrelated to major prognostic factors. The significance of the antigen with respect to etiologic features and prognosis in breast cancer remains to be determined.

Lactosylsphingosine-reactive antibody and CEA in patients with colorectal cancer

The sera of 71 patients with colorectal cancer were examined for lactosylsphingosine-reactive antibody and CEA. Fifteen of those patients were studied repeatedly over extensive periods of time. The antibody was determined by the semiquantitative radioimmunoabsorption technique using lactosylsphingosine-polyacrylamide conjugate and [125I]-labelled anti-human IgG. Excessive antibody levels were invariably found In serum samples of all 39-patients who were examined before or within 2 months after surgery. Serum samples of certain patients became negative for the presence of high antibody levels usually between 3 and 6 months after surgery. This occurred in 13 out of 41 operated patients. The follow-up study revealed that 11 such patients have been free of any signs of cancer relapse up until the time of the follow-Up examination, i.e. for 12-28 months, mean 19 months. In contrast, only 4 out of 28 patients who remained positive for the excessive antibody longer than 2 months after surgery are at present free of the disease. The high antibody levels which persist more than 6 months after surgery are almost always associated with cancer recurrences or metastases. This was true for 21 out of 22 such patients. The high levels of the antibody preceded other signs of cancer relapse, including increased concentrations of CEA in about 40% of the operated patients.

Antibodies reactive with murine mammary tumor virus in sera of patients with breast cancer: geographic and family studies

Sera of patients with breast cancer, of healthy women from the United States, East India, East Africa, and China, and of healthy women of American and Parsi families in which breast cancer occurred in several family members were assayed for levels of antibody reactive with the murine mammary tumor virus (MuMTV) by an enzyme-linked immunosorbent assay. Increased levels of antibody to MuMTV (absorbance greater than or equal to 0.4) were found in sera of 18.6% of American patients with breast cancer and of 2.8% of healthy American women and in 38% of patients from India and 61.9% from East Africa (healthy, 26.9%). In contrast, antibody reactive with MuMTV was found in less than 5 of women with breast cancer from mainland China (healthy Chinese, 5.0%). Differences in serum MuMTV antibody levels between breast cancer patients in the four groups were found to be significant (P less than 0.0001). Studies of two families from the United States and of one Parsi family from India with genetic propensity to breast cancer showed that high levels of antibody to MuMTV were found in 33%, 71%, and 23% of healthy family members, respectively. The antibody to MuMTV was readily absorbed with purified MuMTV and gp52. In contrast, fetal calf serum, murine type c retroviruses, or erythrocytes from various species failed to absorb the antibody.